Tom,
I have a 2011 GT2100 and the headlights on the newer Cubs are still sub standard or worse. I don't operate much after dark but when I do (usually pushing snow) it would be nice to have some decent light output. I am a lighting designer by trade and I know how a reflector should be designed and the bent foil that Cub uses is effectively ineffective. When I was showing an LED lamp to a customer for inside their elevator I noticed the base was a single contact bayonet and operated on 12V, same as my tractor. So I took a couple home to try, put them in and WOW, I must have 4X the light output I had with the factory lamps and because they are LED they will likely last longer than my tractor. I also installed a switch so they wouldn't be on all the time, it still baffles me that MTD had them direct wired to the key!? I would also like to take the advise of some others I've seen post and use silicone to help better seal the lenses. This was a major issue in the snow because snow would make it past the lens, make contact with the hot lamp & blow the bulb. I know your Cub is different but looking at photos of the GT2550 light assembly I think these LED lamps might work.
See photos of the upgrade, factory lamp, LED lamp, LED lamp on, LED lamp & box, switch location.